title: Characteristics of electrode impedance and stimulation efficacy of a chronic cortical implant using novel annulus electrodes in rat motor cortex.
abstract: OBJECTIVE
  Cortical neural prostheses with implanted electrode arrays have been used to restore compromised brain functions but concerns remain regarding their long-term stability and functional performance.
  
  
  METHODS
  Here we report changes in electrode impedance and stimulation thresholds for a custom-designed electrode array implanted in rat motor cortex for up to three months.
  
  
  RESULTS
  The array comprises four 2000 µm long electrodes with a large annular stimulating surface (7860-15700 µm(2)) displaced from the penetrating insulated tip. Compared to pre-implantation in vitro values there were three phases of impedance change: (1) an immediate large increase of impedance by an average of two-fold on implantation; (2) a period of continued impedance increase, albeit with considerable variability, which reached a peak at approximately four weeks post-implantation and remained high over the next two weeks; (3) finally, a period of 5-6 weeks when impedance stabilized at levels close to those seen immediately post-implantation. Impedance could often be temporarily decreased by applying brief trains of current stimulation, used to evoke motor output. The stimulation threshold to induce observable motor behaviour was generally between 75-100 µA, with charge density varying from 48-128 µC cm(-2), consistent with the lower current density generated by electrodes with larger stimulating surface area. No systematic change in thresholds occurred over time, suggesting that device functionality was not compromised by the factors that caused changes in electrode impedance.
  
  
  CONCLUSIONS
  The present results provide support for the use of annulus electrodes in future applications in cortical neural prostheses.
authors.affiliation: Monash Vision Group and Department of Physiology, Monash University, Clayton, VIC 3168, Australia. chunw@anatomy.usyd.edu.au
authors.forename: Chun
authors.initials: C
authors.lastname: Wang
authors.forename: Emma
authors.initials: E
authors.lastname: Brunton
authors.forename: Saman
authors.initials: S
authors.lastname: Haghgooie
authors.forename: Kahli
authors.initials: K
authors.lastname: Cassells
authors.forename: Arthur
authors.initials: A
authors.lastname: Lowery
authors.forename: Ramesh
authors.initials: R
authors.lastname: Rajan
mesh_terms: D000818:Animals; D046690:Deep Brain Stimulation / Q000295:instrumentation*; D017097:Electric Impedance; D004567:Electrodes, Implanted; D004867:Equipment Design; D019544:Equipment Failure Analysis; D019054:Evoked Potentials, Motor / Q000502:physiology*; D008137:Longitudinal Studies; D009044:Motor Cortex / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity